Hyperoxaluria is an important risk factor for kidney stones, approximately 80% of which are primarily composed of calcium oxalate. Hyperoxaluria is typically diagnosed by performing a 24 hour urine collection and levels above 45 mg/day are considered abnormal although, depending on the other urine constituents, the risk of CaOx stones increases when the urinary oxalate level is above 20 mg/dl. It is important to distinguish between hyperoxaluria that results from increased oxalate production (endogenous) and increased oxalate ingestion (enteric).
The classic disease associated with increased oxalate production is primary hyperoxaluria. There are 3 identified types although all result from defects in glycoxylate metabolism leading to oxalate accumulation. At first, the manifestations are primarily renal leading to nephrolithiasis and nephrocalcinosis. However, as the disease progresses, the serum oxalate concentration increases eventually resulting in extra-renal oxalate deposition. Vitamin C is metabolized to oxalate also so that patients with oxalate-containing kidney stones should probably avoid excess vitamin C supplementation as this could increase the risk of stones.
Enteric hyperoxaluria results from increased absorption of oxalate in the large bowel. In general, there are 3 ways in which this might occur:
- Increased dietary oxalate ingestion
- Decreased dietary calcium intake - calcium binds oxalate in the gut and reduces absorption. This is why low calcium diets are not recommended in patients with idiopathic kidney stones. Calcium supplements are a different issue as they may contribute to hypercalciuria and not decrease oxalate ingestion, particularly if they are not taken at mealtimes
- In the setting of malabsorption syndromes and GI disease. This occurs in patients following bariatric surgery, fat malabsorption and inflammatory bowel disease. The mechanism is thought to be related to binding of calcium to fatty acids thus reducing the availability of calcium for oxalate-binding, along with increased large bowel permeability. There have been multiple cases of patients developing severe oxalosis following jejuno-ileal bypass surgery.
The treatment of hyperoxaluria depends on the cause. For all patients, increasing fluid intake is good advice. Some patients with primary hyperoxaluria respond to treatment with pyridoxine which promotes conversion of glycoxylate to glycine instead of oxalate. Recently, a bacterium has been identified that metabolizes oxalate in the gut and this has been
proposed as a potential treatment for hyperoxaluria. Interestingly, antibiotic treatment has been shown to decrease oxalobacter colonization in individuals with peptic ulcer disease.
Of course, all patients with hyperoxaluria should be advised to reduce oxalate consumption in the diet. Foods high in oxalate include spinach, rhubarb, tea, chocolate, star fruit and soy products. A full list can be found
here.
Rhubarb is an interesting case. In the First World War because of the lack of access to fresh vegetables, the British government recommended that families supplement their diets with rhubarb leaves which were not traditionally eaten. It turns out that this was very bad advice. Rhubarb leaves contain considerably more oxalate than the stalks and there was a
flurry of
case reports towards the end of the war detailing cases of oxalate poisoning from rhubarb leaf consumption (
see also and
this). The toxicity of the leaves was probably increased by advice to cool the leaves with soda which increases the solubility of oxalate. Although the MD50 of oxalate would require the ingestion of about 5kg of rhubarb leaves, one could imagine that much lower doses would be toxic in patients with chronic kidney disease.
One last point about oxalate. It is a terminal metabolite and was thought to not have any positive role. However,
recent data have suggested that oxalate is important for chloride transport in the proximal tubule where it acts similarly to formate..
